Mosaic, an urban sketch artist suffering from motor neurone disease, meets an old school friend, Angelfish, at a class reunion. As his illness takes a tragic hold, he asks her to fulfil one wish - to be his girlfriend for just a day.

In 1985, Cheung Yau-ming was one of five miscreants involved in the brutal murder of a white couple. As he was still a juvenile, the court ordered that he be "detained at Her Majesty's pleasure," a clause in British law allowing the government to imprison young offenders for an indefinite period. The film picks up in 1997, with Yau-ming (now 28) being paid a visit by a girl named Cheung Yue-ling. With only six months to go before the Handover, Yau-ming and 22 other prisoners hope to have their sentences determined soon, fearing what might happen should the decision about what to do with them become the province of incoming Chief Executive Tung Chee-hwa.

Narcotics officer Shaolun (Tony Leung Ka Fai), in order to bring down the local drug kingpin King (Michael Wong), forces a female street thug with an equally sketchy past, Coco (Rosamund Kwan) to infiltrate King's organization and provide information. What no one expected was for both men to fall for Coco, and Coco for King.